1. Field of the Invention
The present invention relates to an imaging lens imaging a subject and an imaging apparatus using the imaging lens.
2. Description of Related Art
A compact imaging lens is known which is used in an imaging apparatus, such as an onboard camera or a surveillance camera. An imaging device which includes a compact CCD or CMOS having a large number of pixels for use in such an imaging apparatus, or a compact imaging lens which is assembled with the imaging device is also known.
As an imaging lens used in such an imaging apparatus, imaging lenses described in U.S. Pat. No. 7,023,628, Japanese Patent No. 2599312, JP-A-61-123810, JP-A-2007-279632, and JP-A-2007-139985 are also known in which a cemented lens is used so as to achieve reduction in size.
The imaging lens described in U.S. Pat. No. 7,023,628 is a fast lens having an F number of 2.0, but multiple lenses made of a material having a refractive index larger than 1.9 are used, which causes an increase in costs. The imaging lens described in Japanese Patent No. 2599312 or JP-A-2007-279632 uses an aspheric lens. When this imaging lens is used as a lens for an onboard camera or a lens for a surveillance camera, if a lens is made of plastic, a significant change in performance occurs due to temperature. Further, the use of a lens formed by a glass mold causes an increase in costs. The imaging lens described in JP-A-61-123810 is a lens having an F number of 2.8 to 4.0, and is too slow to use in an onboard camera or a surveillance camera. The lens described in each of U.S. Pat. No. 7,023,628, Japanese Patent No. 2599312, JP-A-61-123810, and JP-A-2007-279632 includes a cemented lens, so there is still room for improvement in terms of environmental resistance.
In the lens described in JP-A-2007-139985, multiple aspheric surfaces are used, and one of the lens surfaces is a diffractive optical surface. For this reason, when a lens is made of plastic, a significant change in performance occurs due to temperature. Further, the use of a lens formed by a glass mold causes an increase in costs.
As an imaging lens used in an onboard camera or a surveillance camera, there is a need for a fast (small F number), compact, and low-cost lens having excellent weather resistance with small deterioration in performance due to a change in temperature so as to be used in the inside of an automobile right on the equator in summer or in the outdoors of a cold area in winter.
In an imaging lens using a cemented lens, however, an adhesive material forming an adhesive layer is placed under a severe environment for a long time and then degenerated or modified, so the optical performance of the imaging lens is deteriorated.
Special processing may be performed so as to allow the cemented lens to be used under a severe environment, but this processing causes an increase in costs.
This problem generally occurs when an imaging lens using a cemented lens is used under a severe environment, regardless of whether it is an onboard camera or a surveillance camera.